Method for urea injection control

ABSTRACT

A urea injection control system ( 10 ) for a vehicle having an exhaust line ( 16 ) in fluid communication with a SCR catalyst ( 18 ) and a urea injector ( 14 ) for injecting urea into the exhaust line includes a controller ( 12 ) for controlling the flow rate of urea injection at the urea injector. The controller ( 12 ) compares an optimum urea flow rate ( 20 ) with a flexible urea flow rate ( 22 ) determined by a flexible flow rate calculator. The controller ( 12 ) selects the minimum value of the optimum urea flow rate ( 20 ) and the flexible urea flow rate ( 22 ), and controls the urea injector ( 14 ) to inject urea at the minimum value.

FIELD OF THE INVENTION

The present invention relates generally to SCR systems, and more particularly, to a control method for injecting urea into the SCR system.

BACKGROUND OF THE INVENTION

Typically, urea selective catalytic reduction systems (urea SCR systems) are used to reduce oxides of Nitrogen (NOx) from engines. Urea SCR systems rely on injection of 32.5% aqueous urea solution into the exhaust line of a vehicle upstream of an SCR catalyst. In the SCR catalyst, the urea is decomposed, and the emission from the catalyst is N₂, H₂O and CO₂.

At the moment of injection, the urea solution temperature is typically close to ambient, and is preferably less than 60° C. The SCR reaction requires gaseous ammonia. To produce the gaseous ammonia, the injected urea solution must be heated, preferably to over 150° C., to evaporate the water and decompose the remaining solid urea into ammonia and isocyanic acid. If the evaporation and the decomposition are not complete, the SCR catalyst performance is reduced due to insufficient availability of reductant. Furthermore, solid deposits of urea can be formed in the exhaust pipe, which can cause complete failure of the SCR system.

The heat required for evaporation and decomposition is provided by the exhaust gas. At low exhaust gas temperatures, the available heat may be insufficient for complete evaporation and decomposition of the urea solution.

Currently, SCR systems are controlled by strategies that impose hard temperature limits. If the exhaust gas temperature becomes less than the set limit, typically 150 to 250° C., the urea injection stops. If the gas temperature is higher than the set limit, the urea injection rate is determined by the control strategy based on NOx flux in the exhaust gas, performance characteristics of the SCR catalyst, and ammonia storage capacity of the SCR catalyst.

However, setting up a hard temperature limit is not desireable. If the temperature limit is set too low, poor evaporation and deposit formation may result. If the temperature limit is set too high, then the control system does not utilize the entire range of temperatures available for the SCR reaction. Being able to utilize the entire range of temperatures available for the SCR reaction is especially important for transient operation when the temperature is generally low and is frequently less than the pre-set limit. An example of such operation is a stop-and-go or city vehicle driving cycle.

SUMMARY OF THE INVENTION

A urea injection control system for a vehicle having an exhaust line in fluid communication with a SCR catalyst and a urea injector for injecting urea into the exhaust line includes a controller for controlling the flow rate of urea injection at the urea injector. The controller compares an optimum urea flow rate with a flexible urea flow rate determined by a flexible flow rate calculator. The controller selects the minimum value of the optimum urea flow rate and the flexible urea flow rate, and controls the urea injector to inject urea at the minimum value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a control method for injecting urea into an SCR system.

DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIG. 1, a flexible limit on the maximum allowable urea solution injection rate (F_(urea) _(—) _(max)) limits the urea dosing rate depending on the exhaust flow rate (F_(exh)) and temperature (T_(exh)), where the limiting rule is determined empirically. The present flexible limit is in contrast to the prior art hard limit where the urea injection is stopped when the exhaust gas temperature does not fall within the temperature limits.

In a urea injection control system 10, a controller 12 controls the flow rate of urea at a urea injector 14, which injects the urea into an exhaust line 16 of a vehicle to decompose upstream of an SCR catalyst 18. The controller 12 has an optimum value 20 and a flexible value calculator 22. The optimum value 20 is determined by the concentration of ammonia required for optimum SCR catalyst performance, and may include factors such as the NOx flux, NOx conversion requirements, SCR kinetics, ammonia storage state, and/or ammonia storage requirements for the particular SCR system. Typically, the optimum value 20 does not take into account the requirements for evaporation of urea solution and decomposition of urea.

The flexible value calculator 22 receives input values of the exhaust gas flow rate (F_(exh)) and temperature (T_(exh)). In certain applications, such as moving vehicles equipped with internal combustion engines, exhaust flow rate measurements are not available, and can be calculated from the values of intake air flow rate and fuel flow rate, or using other parameters available from the engine control unit. For example, oxygen concentration data can be obtained with one or more oxygen sensors or NOx sensors, where one is typically located upstream of the SCR catalyst 18 and is used for feed-forward SCR controller 12, and where another is located in the tailpipe and is used for feedback into the SCR controller 12. The fuel flow rate data can be used to calculate the exhaust gas flow rate.

Subsequently, the maximum allowable urea flow rate, F_(urea) _(—) _(max), is calculated using the formula:

F _(urea) _(—) _(max) =A*F _(exh)(T _(exh) −T _(min))

where, F_(urea) _(—) _(max)=urea solution flow rate

-   -   F_(exh)=exhaust flow rate     -   T_(exh)=exhaust temperature     -   T_(min)=cut-off temperature     -   A=coefficient

Coefficient A and T_(min) can be determined or calibrated experimentally for a given SCR system configuration. It is expected that experimental calibration would be required to account for unique parameters of the particular SCR system, such as heat losses, urea spray parameters, geometry of the part of the SCR system where urea solution injection occurs, evaporation, and geometry of the part of the SCR system where decomposition and mixing with the exhaust gas flow take place.

The coefficient A and T_(min) can also be derived from fundamental thermodynamic correlations as follows. Urea is a 32.5% aqueous solution when injected into the exhaust line of a vehicle upstream of an SCR catalyst 18. The injected urea solution is colder than the temperature of the exhaust gas (T_(exh)), therefore energy is transferred from the exhaust gas to the urea solution. As a result of the energy transfer, the temperature of the exhaust gas (T_(exh)) is cooled. The transferred energy is used to heat up the urea solution, evaporate water, evaporate urea, and decompose urea: (NH₂)₂CO→NH₃+HNCO. It is also assumed that HNCO hydrolysis is slow, and its contribution to the energy balance is negligible.

In the final equilibrium state all components have the same temperature, and the final temperature must ensure evaporation and decomposition of urea. Additionally, the theoretical value that urea decomposition begins at is 132.7° C., and urea decomposition is at maximum within the range of about 130 to 180° C., and preferably at about 150° C. Due to heat losses to the pipe walls, a safety margin may be required. Therefore, the recommended urea dosing rate (F_(urea)) must result in the final mixture temperature (T_(min)) that is always ≧150° C., which provides a factor of safety above the theoretical value of 132.7° C.

The maximum flow of urea solution (F_(urea) _(—) _(max)) can be calculated from the energy balance for any given combination of exhaust flow and exhaust temperature values, using the simplified equation:

F _(urea) _(—) _(max)=0.388F _(exh)(T _(exh) −T _(min))

where, F_(urea) _(—) _(max)=urea solution flow rate, g/hr

-   -   F_(exh)=exhaust flow rate, kg/hr     -   T_(exh)=exhaust temperature, ° C.     -   T_(min)=cut-off temperature, ° C.

It should be appreciated that a different coefficient value will work for different units. Additionally, more precise analysis can result in a somewhat different value, not exactly 0.388, but preferably in the range of about 0.35 to 0.45. It is contemplated that in any case, the coefficient to be used in a vehicle should be adjusted or calibrated empirically, using test data.

After F_(urea) _(—) _(max) is calculated, the urea flow rate (F_(urea)) to the exhaust line 16 is selected at a selector 24 as the minimum of the F_(urea max) and the optimum flow rate (F_(optimum)). The controller 12 controls the urea injector 14 to inject the urea at the flow rate (F_(urea)) as determined by the selector 24. In this way, the injection flow rate (F_(urea)) is determined by the control system 10 as optimum for SCR catalyst performance.

The present control system 10 fine-tunes the urea injection rate (F_(urea)). Such fine-tuning results in better utilization of the temperature range where only a limited amount of urea can be safely injected, and aids in the prevention of undesirable urea deposits in the exhaust system. The proposed invention can be used as a safeguard added to any known urea injection method, and can be implemented in any industry standard controller.

The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope. 

1. A urea injection control system for an exhaust line in fluid communication with a SCR catalyst and a urea injector for injecting urea into the exhaust line, the urea injection control system comprising: a controller for controlling the flow rate of urea injection at the urea injector, wherein the controller compares an optimum urea flow rate with a flexible urea flow rate determined by a flexible flow rate calculator, wherein the controller selects the minimum value of the optimum urea flow rate and the flexible urea flow rate, and controls the urea injector to inject urea at the minimum value.
 2. The urea injection control system of claim 1 wherein the flexible value calculator receives inputs of exhaust flow rate and exhaust temperature.
 3. The urea injection control system of claim 2 wherein the flexible urea flow rate calculator determines the flexible urea flow rate with the equation: F _(urea) _(—) _(max) =A*F _(exh)(T _(exh) −T _(min)) where, F_(urea) _(—) _(max)=flexible urea flow rate, g/hr F_(exh)=exhaust flow rate, kg/hr T_(exh)=exhaust temperature, ° C. T_(min)=cut-off temperature, ° C. A=coefficient
 4. The urea injection control system of claim 3 wherein the coefficient A is between 0.35 and 0.45.
 5. The urea injection control system of claim 3 wherein Tmin is in the range of 130 to 180° C.
 6. The urea injection control system of claim 1 wherein the flexible value calculator receives inputs of at least one of intake airflow, fuel flow rate, oxygen concentration, and fuel flow rate to calculate the exhaust flow rate and the exhaust temperature.
 7. The urea injection control system of claim 6 wherein the flexible urea flow rate calculator determines the flexible urea flow rate with the equation: F _(urea) _(—) _(max) =A*F _(exh)(T _(exh) −T _(min)) where, F_(urea) _(—) _(max)=flexible urea flow rate, g/hr F_(exh)=exhaust flow rate, kg/hr T_(exh)=exhaust temperature, ° C. T_(min)=cut-off temperature, ° C. A=coefficient
 8. The urea injection control system of claim 7 wherein the coefficient A is between 0.35 and 0.45.
 9. The urea injection control system of claim 7 wherein Tmin is in the range of 130 to 180° C.
 10. The urea injection control system of claim 1 wherein the controller further comprises a selector for selecting the minimum value of the optimum urea flow rate and the flexible urea flow rate, and for controlling the urea injector to inject urea at the minimum value.
 11. A urea injection control system for an exhaust line in fluid communication with a SCR catalyst and a urea injector for injecting urea into the exhaust line, the urea injection control system comprising: a controller for controlling the flow rate of urea injection at the urea injector, wherein the controller compares an optimum urea flow rate with flexible urea flow rate calculator, wherein the flexible urea flow rate calculator receives inputs of exhaust flow rate and exhaust temperature; and a selector at the controller that selects the minimum value of the optimum flow rate and the flexible flow rate, wherein the urea injector is instructed by the selector to inject urea at the minimum value.
 12. The urea injection control system of claim 11 wherein the flexible urea flow rate calculator determines the flexible urea flow rate with the equation: F _(urea) _(—) _(max) =A*F _(exh)(T _(exh) −T _(min)) where, F_(urea) _(—) _(max)=flexible urea flow rate, g/hr F_(exh)=exhaust flow rate, kg/hr T_(exh)=exhaust temperature, ° C. T_(min)=cut-off temperature, ° C. A=coefficient
 13. The urea injection control system of claim 12 wherein the coefficient A is between 0.35 and 0.45.
 14. The urea injection control system of claim 12 wherein Tmin is in the range of 130 to 180° C. 